This chapter delves into the impact of Plasma Electrolytic Oxidation (PEO) on the mechanical properties of various metallic systems, focusing on aluminum, magnesium, and titanium alloys. PEO is a surface treatment process that forms a hard, ceramic-like oxide layer on metal surfaces, significantly enhancing properties such as hardness, wear resistance, and corrosion resistance. The chapter begins with an introduction to the PEO process, highlighting its versatility and the ability to tailor coatings for specific applications by adjusting process parameters and electrolyte compositions. It then explores the effects of PEO on surface hardness and wear resistance, emphasizing the formation of dense oxide layers and the incorporation of nanoparticles to further enhance these properties. The discussion extends to the challenges posed by microcracks and pores within the oxide layer, which can compromise mechanical performance, and the various posttreatment processes developed to mitigate these issues. Additionally, the chapter addresses the influence of PEO on stress corrosion cracking and corrosion fatigue, underscoring the importance of understanding the interplay between mechanical stress and corrosive environments. By examining the mechanisms and factors that influence PEO coatings, this chapter provides valuable insights for optimizing the process to develop advanced materials with superior mechanical properties for a wide range of industrial applications.